1. Field of the Invention
The invention relates to a thin film electroluminescent (EL) device, and more partiucularly to an improvement in luminance of an electroluminescent device that emits in response to the application of an AC electric field.
2. Description of the Prior Art
Conventionally, since the development of electroluminescence (EL) that can be obtained by applying an AC electric field to zinc sulfide (ZnS), various investigations have been made concerning luminescent center elements to be doped, fluorescent host materials and the like. Particularly, a thin film electroluminescent device in the so-called double insulating structure is so constructed that a ZnS thin film containing manganese (Mn) as a luminescent center element is first sandwiched with insulating layers. It is further covered at both sides with electrodes, at least one of which is transparent. Such an electroluminescent device, as a thin and lightweight display, has been rather broadly used in a portable computer and a measuring device. Recently, luminescent devices having a larger capacity, such as 720.times.400 dots (about 300,000 picture elements) corresponding to enlargement of displays, have been marketed.
A basic structure of a double insulating thin film electroluminesenct device is shown in FIG. 3. The device is so constructed that a transparent electrode 2, a first insulating layer 3, a luminescent layer 4, a second insulating layer 5, and a back electrode 6 are deposited in the mentioned order on a transparent substrate 1, made of glass and the like, by thin film deposition technology such as electronbeam deposition, a sputtering process or the like. Between the transparent electrode 2 and the back electrode 6 is connected an AC power source 7. When voltage is applied which is more than a threshold voltage (Vth), it shows a broad emitting spectrum having peak length wave at 585 nm, inherent in Mn luminescent center. The emission color is yellowish-orange.
However, thin film electroluminescent devices, which are practically used now, are only the above-mentioned ones using ZnS:Mn luminescent layer, which achieve only monochrome display. Accordingly, it is believed that a thin film electroluminescent device enabling multi-color display has not been realized.
Thus, it is desirable that a luminescent layer emitting various colors other than yellowish-orange, particularly, three primary colors of red, green and blue, be developed to provide a multi-color or full-color display thin film electroluminescent device. In this regard, a ZnS:Ln luminescent layer, using rare earth elements (called hereinafter Ln) as a luminescent center element, shows a luminescent spectrum ingerent in Ln, due to the 4 f electron transition. Thus, a red color emission can be obtained by Sm and Eu, green by Tb, and blue by Tm. This has been reported in "Electroluminescence of ZnS Lumocen Devices Containing Rare-Earth and Transition-Metal Fluorides" J.Appl.Phys.Vol. 40, No. 6, p.p. 2512-2519 (1969); "Color Electrolumnescent Devices Prepared by Metal Organic Chemical Vapor Deposition" JAPAN DISPLAY '86, p.p 254-257; and multicolor "Electroluminescent ZnS Thin Films Doped with Rare Earth Fluorides" phys. stat. sol (a) 88,713 (1985).
Although investigations have been actively made, when using a ZnS:Ln luminescent layer, it cannot achieve a sufficient brightness in each of the emission colors and is thus unable to be used practically.
Also, a high brightness green-emitting electroluminescent device using a ZnS:Tb luminescent layer, fabricated by means of sputtering method/has been reported ["High-brightness green-emitting electrolumine- scent devices with ZnS:Tb,F active layers", Appl. Phys. Lett. 48(23), 9 June 1986].
However, it has not been known that ZnS:Ln luminescent layers other than ZnS:Tb luminescent layer show high luminance.